1. Field of the Invention
This invention relates to a multi-layer information medium which has at least two information-storing layers such as recording layers.
2. Prior Art
There is a growing need for an optical disk having a higher density and a higher capacity. DVD (Digital Versatile Disk) is already commercially available, and the DVD has a storage capacity of about 4.7 GB per single side which is about seven times larger than the compact disk. Technologies enabling further increase in the amount of information recorded have been actively developed.
Technologies that have been used for increasing the recording capacity of an optical disk include use of a recording/reading beam having a shorter wavelength, use of an objective lens having a higher NA (numerical aperture) in the optical system irradiating the recording/reading beam, increase in the number of recording layers, and multi-value recording. Among these, three-dimensional recording by increasing the number of recording layers enables remarkable increase in the recording capacity at low cost compared to the use of a shorter wavelength or use of a lens with a higher NA. The multi-layer information medium wherein the three-dimensional recording has been enabled is described, for example, in Japanese Patent Application Kokai (JP-A) 198709/1997, and JP-A 255374/1996 discloses a medium wherein a rewritable information storage layer and a read only information storage layer are laminated.
A multi-layer information medium is provided with a plurality of data layers including recordable recording layers and/or read only layers formed with pits, and accordingly, recording and reading of a multi-layer information medium require information for identifying each particular layer. When a layer has its own optimal recording/reading conditions or servo system unique to the specific layer which may be different from other layers, such information also needs to be recorded in the medium. Such information is herein referred to as xe2x80x9clayer-specific informationxe2x80x9d. In spite of such situation, no valuable proposals seems to have been so far made as to how such layer-specific information should be recorded. Besides, a multi-layer information medium has a plurality of data layers which are to be read, and there is a risk that the step of reading the layer-specific information results in the delay of the access to the information recorded in the data layer.
A multi-layer recording medium also suffers from the problem as described below. In the case of the medium having a single recording layer formed on a substrate, the shape of the grooves (guide grooves) formed in the resin substrate will be transferred to the recording layer. In contrast, in the case of a medium wherein two or more recording layers are formed on the substrate with an intervening relatively thick transparent layer between the recording layers, it is quite difficult to transfer the shape of the grooves formed in the substrate to all of the recording layers since the groove depth is about 100 nm at most for optical reasons while the distance between the recording layers is a more than such groove depth. As a consequence, formation of the grooves in the transparent resin layer by photopolymerization (2P) process will be required as described, for example, in the JP-A 198709/1997 and an eminent increase in the production cost is invited.
In view of such situation, an object of the present invention is to provide a multi-layer information medium wherein the layer-specific information has been adequately recorded, and delay in the access to the recorded data associated with the reading of the layer-specific information is suppressed. Another object of the invention is to provide such multi-layer information medium at low cost.
Such objects are attained by the present invention as described in (1) to (12), below.
(1) An optical information medium having at least two information-storing layers each storing recorded information and/or tracking servo information, wherein at least one of said information-storing layers is a data layer storing recorded information and there is at least one information-storing layer which is recorded or read by the recording beam or the reading beam which has passed through other information-storing layer(s), wherein
said data layer has layer-specific information unique to the particular data layer recorded therein.
(2) An optical information medium according to the above 1 wherein said layer-specific information includes at least one of information which identifies the particular data layer, recording conditions for the particular data layer, reading conditions for the particular data layer, modulation system of the information recorded in the particular data layer, and servo information required for the servo control of the particular data layer.
(3) An optical information medium according to the above (1) or (2) wherein said layer-specific information can be read with no tracking servo operation.
(4) An optical information medium according to any one of the above (1) to (3) wherein said layer-specific information is recorded in a layer-specific-information-recorded area, and said layer-specific-information-recorded area has a width in the direction perpendicular to the recording track of 5 xcexcm to 5 mm, and said layer-specific information is readable at any location along its width.
(5) An optical information medium according to any one of the above (1) to (4) wherein said layer-specific-information-recorded area is in the form of a bar code comprising low reflectivity areas and high reflectivity areas alternately arranged in the direction of the recording track.
(6) An optical information medium according to the above (5) wherein said data layer is a recording layer and/or a reflective layer, and said low reflectivity areas or said high reflectivity areas are defined by absence of the recording layer and/or the reflective layer.
(7) An optical information medium according to the above (5) or (6) wherein at least one of said data layer(s) is a phase change recording layer, and said low reflectivity areas or said high reflectivity areas are defined by changing the crystalline state of said recording layer.
(8) An optical information medium according to any one of the above (5) to (7) wherein said data layer is a recording layer and/or a reflective layer, and said low reflectivity areas or said high reflectivity areas are defined by providing projections and/or depressions in the recording layer and/or the reflective layer.
(9) An optical information medium according to any one of the above (1) to (8) wherein said layer-specific information includes optimal linear velocity of the medium in the reading, and the medium includes a plurality of data layers each having its own optimal linear velocity.
(10) An optical information medium according to any one of the above (1) to (9) wherein said layer-specific information includes optimal irradiation pattern of the recording beam, and the medium includes a plurality of data layers each having its own optimal irradiation pattern.
(11) An optical information medium according to any one of the above (1) to (10) wherein the medium includes a plurality of data layers each having different thickness.
(12) An optical information medium according to the above (11) wherein at least two phase change recording layers are included in said data layers, and the thicker phase change recording layer has a composition exhibiting lower crystallization speed.